1. Field of the Invention
The present invention relates to infrared semiconductor lasers and in particular to such lasers for use in the range of about 1.6 to about 2.5 .mu.m.
2. Description of the Prior Art
Single-mode semiconductor lasers, with radiation in the mid-infrared region in the range of about 1.6 to about 2.5 .mu.m, are required for light detection and ranging (LIDAR) applications employing atmospheric transmission windows for remote sensing of atmospheric gasses which absorb radiation strongly in this range.
Lasers in the mid-infrared range using gallium antimonide (GaSb) alloy systems, have been demonstrated at discrete wavelengths within a spectral region from about 1.8 to about 4 .mu.m. Structures using the GaSb alloy system are, however, difficult to work with, have low thermal conductivities and dissociate in the temperature ranges used for conventional semiconductor laser fabrication techniques such as regrowth GaSb structures cannot therefore easily be manufactured in complex configurations such as distributed feedback (DFB) lasers or buried mesa configurations.
Semiconductor lasers using the indium phosphide (InP) systems are well known and easily worked, but conventional designs for such lasers using such are limited to a maximum wavelength of about 1.6 .mu.m for lattice matched systems InP alloy systems are usually used at wavelengths of 1.3 and 1.55 .mu.m, in lattice matched and lattice mismatched configurations, to match the characteristics of glass fibers used in fiber optic transmission systems.
The operating wavelength of semiconductor lasers using the InP alloy system may be lengthened by the use of strained active layers in which the active layers are not lattice matched, that is, are lattice mismatched, to the substrate and other epitaxial layers. The lattice mismatch may be controlled by the relative percentage composition of the indium and gallium. For example, In.sub.0.53 Ga.sub.0.47 As layers have a sufficiently similar crystal structure to the InP substrate to be lattice matched thereto, while In.sub.0.75 Ga.sub.0.25 As layers have sufficiently different crystal structures to be mismatched with an InP substrate and form strained layers.
As a strained active layer is made thicker, the bandgap energy decreases, increasing the resonant wavelength. Multiple quantum well structures may then be used to increase the gain or efficiency of such chip lasers. In particular, strained layer, multiple quantum well (SL-MQW) configurations are known for use at 1.55 .mu.m to efficient produce laser light sources for fiber optic cables used in telecommunications applications.
There is a limit, however, known as the critical limit, beyond which the thickness of the strained active layer or layers may not be further increased to increase the operating wavelength of semiconductor lasers. The lasers will not operate for very long, if at all, beyond the critical limit.
What is needed are techniques for conveniently building semiconductor lasers with known fabrication techniques that will reliably produce laser output in the range of about 1.6 to about 2.5 .mu.m.